Duoplasmatron-type ion source including a non-magnetic anode and magnetic extractor electrode



June 16, 1964 N. B. BROOKS ETAL 3,137,801

DUOPLASMATRON-TYPE ION SOURCE INCLUDING A NON-MAGNETIC ANODE AND MAGNETIC EXTRACTOR ELECTRODE Filed Sept. 22, 1960 P/Q/OP ART 0 IIIIIIIIIII/I/IIIIIII United States Patent Ofii ce 3,137,801 Patented June 16,1964

3 137 801 DUorLAsMATaoN-rirnrors SOURCE INCLUD- ING A NON-MAGNETIC ANODE AND MAG- NETIC EXTRACTUR ELECTRODE Norman E. Brooks, Bedford, and Bertram S. Quintal, Peabody, Mass, assignors to High Voltage Engineering Corporation, Burlington, Mass, a corporation of Massachusetts Filed Sept. 22, 1960, Ser. No. 57,825 1 Claim. (Cl. 31363) This invention relates to duoplasmatron-type ion sources and in particular to a novel magnetic circuit for such an ion source.

In the conventional duoplasmatron-type ion source the magnetic circuit includes the magnetic grid and the grounded anode, both of which are of ferromagnetic material, but does not extend beyond the grounded anode. The difiiculty with this arrangement is that at high ionbeam currents space charge in the ion beam causes beam blow-up between the grounded anode and the extractor electrode with the result that the extractor electrodes have been ruined by ion bombardment. In accordance with the invention, the grounded anode is made of aluminum, stainless steel, copper or other paramagnetic material or non-magnetic material, and the extractor electrode is made of magnetic material such as iron, so that the magnetic circuit does not run through the grounded anode but instead runs across it and through the extractor electrode. Previously the extractor electrode had been made of stainless steel or other non-magnetic material. The result of the invention is to provide a magnetic field for confining the beam as it is extracted between the grounded anode and the extractor electrode.

The invention may best be understood from the following detailed description thereof, having reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic view of a conventional duoplasmatron-type ion source; and

FIG. 2 is a diagrammatic view of an ion source embodying the invention.

The conventional duoplasmatron ion source is more fully described in the following material: Manfred von Ardenne, Tabellen der Elektronenphysik Ionenphysik und Ubermikroskopie (Deutscher Verlag der Wissenschafton, Berlin, 1956), page 544; The Review of Scientific Instruments, Volume 30, pages 694 to 699, August 1959; Proceedings of the IRE, Volume 40, page 645, June 1952; British Patent 680,347; British Patent 680,350.

Referring to FIG. 1 of the drawing, in the conventional duoplasmatron type ion source there are two concentrated regions of plasma, shown in FIG. 1 at 1 and 2.. One such region, shown at 1 in FIG. 1, is produced by providing a restricted aperture in the vicinity of the anode between the cathode and the anode and the other of which is produced by means of an inhomogeoneous magnetic field of a pole face lens. Electrons produced at a cathode 3 are accelerated towards an anode 4 by the electric field which is produced by impressing a potential difierence of, say, 500 volts therebetween by means of a voltage source 5. The anode 4 is provided with an emission aperture 6 through which ions are emitted. A first magnetic pole face 7 is provided between the cathode 3 and the anode 4 in the vicinity of the anode 4 and acts as an intermediate electrode. This magnet pole face 7 has a capillary 8 through it which may have a diameter, for example, of about millimeters. The small aperture of this intermediate electrode 7, which is maintained at about 70 to 180 volts by the potential divider 9, serves to confine the discharge, and in the vicinity of the anode 4, the plasma takes the form of a bubble 1 which is bounded by an electrical double layer. The length of the capillary 8 must be so measured that the discharge mechanism in the region 1 of the double layer bubble is not disturbed by the fringing magnetic field of the pole face lens 7, which is maintained at about 70 to 180 volts by the potential divider 9. In this double layer the electrons of the thinner plasma on the side of the cathode 3 are accelerated and in the geometry shown are focused exactly on the region in front of the capillary 8. As a result, a particularly thick plasma is produced in the region 1 in front of the capillary 8 in the intermediate electrode 7. This effect is that of the unoplasmatron which is described in the book by von Ardenne referred to above at page 543 and also in British Patent No. 795,766.

Additional improvement is provided by the inhomogeneous field of the pole face lens comprised by the magnetic pole face 7 and the anode 4 which acts as a second magnetic pole face, a tungsten insert 10 being provided at the center of the anode 4 because of ion bombardment. The eifect of the inhomogeneous magnetic field is described on pages 124 and 125 of the book by von Ardenne referred to above, and the effect has been described as the production of a magnetic mirror field which acts to reflux the electrons so that escape is possible only very near to the axis. An extraction electrode 11 is maintained at a potential of -50 to 70 kilovolts; the extracting field does not operate on the plasma itself, but only on those ions which have escape from the plasma through the anode emission aperture 6.

The magnetic circuit in the device of FIG. 1 includes the intermediate electrode 7 and the anode 4 and is completed by an appropriate return path such as return path 12 of FIG. 2. These three members are therefore made of magnetic material and the necessary magnetic flux may be furnished either through the use of permanent magnets or by providing a winding as shown at 13 in FIG. 2 which may, for example, surround the intermediate elec trode 7. The magnetic flux is therefore confined to the magnetic material with the exception of the gap between the intermediate electrode 7 and the anode 4 and consequently there is no magnetic flux between the anode 4 and the extractor electrode 11. Space charge in the ion beam in this latter region causes the cross section of the ion beam to increase as it approaches the extractor electrode 11 with resultant hazard that the outermost ions in the ion beam will strike the extractor electrode 11 thereby causing damage.

Referring now to FIG. 2, the ion source therein shown is essentially the same as that shown in FIG. 1, except that the anode 4' instead of being made of magnetic material is now made of non-magnetic or para-magnetic material such as aluminum, stainless steel, copper, and the extractor electrode 11' is made of magnetic material such as iron. As a result, the magnetic circuit now includes the intermediate electrode 7, the extractor electrode 11', and the magnetic return path 12 and 12'. There is now an axial magnetic field not only in the region between the intermediate electrode 7 and the anode 4', but also between the anode 4' and the extractor electrode 11' so that the ion beam is subjected to a confining action until it has reached and passed through the aperture in the extraction electrode 11'.

In the embodiment of the invention shown in FIG. 2, additional reluctance has been introduced into the magnetic circuit by virtue of the gap between the two halves of the return path 12 and 12'. However, the area of this gap is relatively large so that the additional reluctance introduced thereby is not objectionable.

As hereinbefore stated, the invention provides an axial magnetic field beyond the anode and thus confines the ions on their trajectory from the anode to the extractor electrode.

for purposes of limitation, the scope of the invention being 5 set forth in the following claims:

We claim:

An ion source comprising in combination an electronemitting cathode, an apertured intermediate electrode, a non-magnetic apertured anode, and an apertured extraction electrode serially arranged in axial alignment, means for producing an axial magnetic field, a magnetic circuit including said apertured intermediate electrode, said apertured extraction electrode and a magnetic return path between said intermediate electrode and said extraction 15 electrode in spaced relationship from the apertures thereof, said magnetic return path including a gap between said intermediate electrode and said extraction electrode in which said axial magnetic field extends on both sides of said anode, means for impressing a potential dilference between said anode and said cathode of sufiicient magnitude to maintain a plasma in the region therebetween, and means for impressing a potential difference between said anode and said extraction electrode of sufiicient magnitude to remove ions from the region of the aperture in said anode, said ions being therefore focused by said axial magnetic field not only in the plasma region but also in 10 the extraction region.

References Cited in the file of this patent UNITED STATES PATENTS 2,759,117 Hasbrouck Aug. 14, 1956 2,831,134 Reifenschweiler Apr. 15, 1958 2,934,665 Zeigler Apr. 26, 1960 2,975,277 Von Ardenne Mar. 14, 1961 

